1. Technical Field
This invention relates generally to pressure vacuum relief valves, and, more specifically, to a non-sticking valve seat and pallet assembly for use in a pressure vacuum relief valve for low pressure applications.
2. Background
Pressure vacuum relief valves are installed on storage tanks and the like, particularly hydrocarbon storage tanks, to provide protection against positive or vacuum over-pressure and to prevent air intake, evaporative losses of product, and the release of odorous and potentially explosive vapors. They are sometimes called breather vents, tank vents, and pressure vacuum release vents. Pressure vacuum relief valves are designed and utilized for both vent to atmosphere and pipe away applications.
Pressure vacuum relief valves used in the oil and gas industry generally include a valve seat, typically a ledge formed around a valve orifice as part of a cast valve housing, and a pallet or disc designed to sealingly engage the valve seat. The pallet maintains a seal until system pressure or vacuum exceeds the set pressure of the valve. When over-pressure occurs the pallet lifts, breaking the seal between the valve seat and pallet, allowing vapors to pass through the valve orifice and relieving the pressure or vacuum build-up. The valve reseals upon relief and remains sealed.
Current systems use several upstanding guide pins spaced around the periphery of the pallet above the top surface of the pallet (above the process) to guide the pallet when it raises or lowers. A stem extending above the top surface of the pallet is received in a receptacle space cast into a valve housing lid and keeps the pallet from lifting too far off the valve seat.
Sludge and condensate build-up, along with temperature extremes, can interfere with valve operation. One problem with current systems, especially prevalent in low pressure applications, is the sticking or freezing of the valve seat and pallet. The ledge on which the pallet comes to rest and the multiple points of contact between the periphery of the pallet and the tangential guide pins provide a significant surface area for sticking or freezing to occur.
Another problem caused by contemporary design is valve wear and flutter. Movement of the pallet against the valve seat and guide pins eventually wears down the component elements of the valve and damages the integrity of the seal. Compounding this problem is the difficulty in replacing valve components in the field. If a valve seat is machined into the valve housing, the entire housing must be replaced after the seat becomes worn.
It is thus the object of the present invention to provide a valve seat and pallet assembly for use in pressure vacuum release valves that resists sludge and condensate build-up and otherwise prevents valve sticking and freezing.
It is another object that the valve seat and pallet assembly provide a smooth valve stroke during operation to reduce valve wear and flutter.
It is a further object that the valve components be field replaceable without the requirement for special tools or complex procedures.